Emergency energy storage devices that supply electric energy to energy consumers find widespread use. As a rule, the energy consumers that need to be supplied with emergency power are safety-relevant. Examples of such safety-relevant energy consumers are especially motors in passenger elevators or the pitch system in wind turbines.
If the external power grid fails, as can happen, for example, in the case of a building fire, the elevators have to be able to proceed to the nearest floor and then the doors have to open without receiving power from the external power grid so that the persons present in the elevator can be brought to safety.
Modern wind turbines are normally provided with electric pitch systems that have at least one electric motor—referred to as a pitch motor—for each rotor blade. By rotating the rotor blades around their longitudinal axes, such pitch systems regulate the position of the rotor blades relative to the wind and they are often the only reliable way to bring the rotor of a wind turbine to a standstill. This is achieved in that the pitch system rotates the rotor blades into the so-called feathered position and the rotor comes to a standstill since it is no longer being driven by the wind. Energy is normally supplied to the pitch system by the power grid into which the wind turbine also feeds the power it has generated. Failure of the power grid can give rise to a hazardous situation, for example, if the winds pick up, since the rotational speed of the rotor of the wind turbine might exceed the maximum permissible value and cause damage to the wind turbine or injury to persons who are in the vicinity.
In order to prevent such a hazardous situation, even if the power grid fails, it must be possible to move the rotor blades into the feathering position, even when the external power grid is not supplying energy to the pitch system. Towards this end, pitch systems known from the state of the art are equipped with one or more emergency energy storage devices which, in case of a power grid failure, ensure that energy is supplied to the pitch system, thus guaranteeing the functionality of the pitch system, at least until the rotor blades have been moved into the safe feathering position.
One problem encountered when electric energy is provided by an emergency energy storage device is that, as a rule, such emergency energy storage devices undergo an ageing process. This means that, after a certain service life, the emergency energy storage devices no longer meet the same performance parameters as they did in the beginning.
In particular, the maximum amount of energy that can be stored in an emergency energy storage device decreases over the course of time. The voltage or the maximum output are also parameters that might be affected. The rate and extent of the ageing process are dependent, among other things, on the ambient temperature and on the voltage at which the emergency energy storage device is charged.
In order to meet the desired performance parameters until the end of a selected time span, that is to say, until the end of the service life of the emergency energy storage device, and thus to ensure, for instance, the safety of the wind turbine, the emergency energy storage devices known from the state of the art are dimensioned in such a way that the desired performance parameters are exceeded at the beginning of the selected time span to such an extent that the desired performance parameters are still met at the end of the selected time span, in spite of the decline caused by ageing. This necessary over-dimensioning of the emergency energy storage device raises the costs accordingly. When it comes to emergency energy storage devices in wind turbines, it is especially the performance parameters relating to the maximum storable amount of energy that are over-dimensioned. Moreover, during the operation of the wind turbine, the emergency energy storage device is charged in such a way that the maximum amount of energy that is possible at a given point in time is always stored in the emergency energy storage device. This is done in order to ensure that sufficient energy is stored in the emergency energy storage devices at all times.
Before this backdrop, the objective of the invention is to put forward a method for charging an emergency energy storage device that takes into account and minimizes the ageing of the emergency energy storage device.